Because of their excellent biocompatibility and adjustable degradation rate, biodegradable and biocompatible polymers have been extensively studied as drug controlled release carriers. Besides selecting the optimal polymer for specific drug delivery purpose, how to incorporate drugs efficiently into polymers is another important issue. Polymers have been prepared into different physical forms, such as micelles, nanoparticles, microspheres, films, gels, to facilitate the encapsulation and controlled release of drugs (see Uhrich, K. E., et al., Chem. Rev. 1999, 99, 3181-3198). In spite of all the advances made, there is much to be further improved. First, most micelles, microspheres or particles often have low drug loading efficiency, as low as a few percent in some cases. Second, many formulation technologies need the assistance of organic solvents to dissolve drugs during dosage preparation and processing. However, the use of organic solvents might be detrimental to delicate drugs such as proteins or peptides, and the residual solvent could be a safety concern for human use. The emergence of injectable biodegradable polymers holds promise in solving these problems (see Amsden, B. Macrom. Biosci. 2010, 10, 825-835; and Heller, J, et al., Adv. Drug Deliv. Rev. 2002, 54, 1015-1039). Drugs could be mixed with or dissolved in viscous semi-solid polymers directly at room temperature with loading efficiency of 100%, and the facile incorporation is especially beneficial for those thermally sensitive or solvent-sensitive drugs such as proteins and peptides. The injectability of semi-solid polymers and administration via minimally invasive means is another advantage.
Currently there remains is a large unmet need for better delivery systems to achieve sustained release of drugs and other biologically active agents at prescribed durations.